Adding outlets to unfinished basement - recommendations

Looking to add an outlet to each of four different walls to an unfinished basement room. I plan to upgrade to run #12 Romex (already have) to that room from a circuit that just has a light on it. I may need to upgrade the breaker to a 20A and the wire to the light to #12. Full access to ceiling joists is available. I plan to install a GFCI outlet nearest the circuit breaker box and want to chain the other outlets to it. The walls are stone or brick and mortar.

Should I install junction boxes above each drop to the outlet boxes?

Should I use metal conduit or PVC? Same question for boxes.

Conduit size?

Fittings needed?

To have the GFCI protect subsequent outlets do I only need to run an additional hot wire down the conduit making the neutral and ground connections in the ceiling junction box or could I just run 2 Romex down the conduit drop or should everything after the first ceiling junction box be THHN?

It's good that you recognized that receptacles in unfinished spaces need to be GFCI protected (and you may need arc fault protection as well). In many jurisdictions new or extensions of existing circuits need to be permitted and inspected. Your local building code will also determine suitable wiring means but generally speaking NM-B (aka Romex) needs to be protected, which means you can't run it down the surface of a block or concrete wall. NM-B can be run in conduit for short distances for physical protection.

I would probably use a combo GFCI/AFCI breaker, come out of the junction box to some central point in the ceiling. From there, go out to each of the locations where you want to place a receptacle. As you go down the wall, sheath the NM-B in conduit for physical protection.

In addition to the great advice from Adam, let me address each of your questions individually.

1. Should I install junction boxes above each drop to the outlet boxes?

You do not need to put junction boxes above each drop. If you really are trying to pull NM-B through conduit, then you may find it easier to add the boxes. I personally would not use the NM-B wire just because I have it sitting around. As Adam said, you must use conduit for exposed wiring. Junction boxes should be placed where they will remain accessible, and where it makes sense based on what branching off you want to do.

2. Should I use metal conduit or PVC? Same question for boxes.

Metal conduit. Hopefully EMT will meet code requirements. PVC conduit is for outside under ground burial. It is not used inside the home. As for boxes, use metal ones wherever you use metal conduit.

3. Conduit size?

You can use 1/2" conduit if you use standard wires. Trying to pull NM-B cable would make me switch to 3/4" conduit just to make life easier.

4. Fittings needed?

Once again, the fittings you use depends on the wire you choose to use.

5. To have the GFCI
protect subsequent outlets do I only need to run an additional hot wire
down the conduit making the neutral and ground connections in the
ceiling junction box or could I just run 2 Romex down the conduit drop?

In order for a GFCI to work properly with additional outlets, you will need to utilize the "load" terminals on the GFCI outlet to feed all additional outlets. As such you would need to have 2 sets of hot and neutral wires running. I would just rather put in a GFCI outlet at every place I needed a new one. It will not make much of a cost difference, will make your life easier for wiring, and there is a third benefit also: Every added outlet and wire run on the load side of a GFCI makes it more sensitive to hot/neutral imbalance. This can make troubleshooting maddening if the outlet trips over and over again.

6. Should everything after the first ceiling junction box be THHN?

If you think pulling one MN-B cable through conduit is fun, wait until you try 2 at once! I would save your Romex cable for something else, and use 12 gauge THHN/THWN wire. If you decide to utilize the load side of a GFCI to protect downstream standard outlets, keep in mind that the hot wires should be purple, and the neutral wires grey instead of the standard black and white. This color change is done so that any subsequent electrician working on a downstream outlet will know immediately that it is being fed from a GFCI load side, even if the wall plate stickers fall off.

One last recommendation: I would purchase a 20 amp arc fault circuit breaker to protect all the circuit wiring as well as anything plugged into outlets. Then the purchase of simple GFCI breakers will be less expensive. This should provide maximum protection for the least cost.